Steel has been used by mankind for at least 3,000 years and are widely utilized in industry comprising over 80% by weight of all metallic alloys in industrial use. Existing steel technology is based on manipulating the eutectoid transformation. The first step is to heat up the alloy into the single phase region (austenite) and then cool or quench the steel at various cooling rates to form multiphase structures which are often combinations of ferrite, austenite, and cementite. Depending on how the steel is cooled, a wide variety of characteristic microstructures (i.e. pearlite, bainite, and martensite) can be obtained with a wide range of properties. This manipulation of the eutectoid transformation has resulted in the wide variety of steels available currently.
Currently, there are over 25,000 worldwide equivalents in 51 different ferrous alloy metal groups. For steel, which is produced in sheet form, broad classifications may be employed based on tensile strength characteristics. Low Strength Steels (LSS) may be defined as exhibiting tensile strengths less than 270 MPa and include such types as interstitial free and mild steels. High-Strength Steels (HSS) may be defined as exhibiting tensile strengths from 270 to 700 MPa and include such types as high strength low alloy, high strength interstitial free and bake hardenable steels. Advanced High-Strength Steels (AHSS) steels may be defined as exhibiting tensile strengths greater than 700 MPa and include such types as martensitic steels (MS), dual phase (DP) steels, transformation induced plasticity (TRIP) steels, and complex phase (CP) steels. As the strength level increases, the ductility of the steel generally decreases. For example, LSS, HSS and AHSS may indicate tensile elongations at levels of 25% to 55%, 10% to 45% and 4% to 30%, respectively.
Steel material production in the United States is currently about 100 million tons per year and worth about $75 billion. According to the American Iron and Steel Institute, 24% of the US steel production is utilized in the auto industry. Total steel in the average 2010 vehicle was about 60%. New advanced high-strength steels (AHSS) account for 17% of the vehicle and this is expected to grow up to 300% by the year 2020. [American Iron and Steel Institute. (2013). Profile 2013. Washington, D.C.]
Continuous casting, also called strand casting, is one of the most commonly used casting process for steel production. It is the process whereby molten metal is solidified into a “semifinished” billet, bloom, or slab for subsequent rolling in the finishing mills (FIG. 1). Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary molds to form ingots. Since then, “continuous casting” has evolved to achieve improved yield, quality, productivity and cost efficiency. It allows for lower-cost production of metal sections with better quality, due to the inherently lower costs of continuous, standardized production of a product, as well as providing increased control over the process through automation. This process is used most frequently to cast steel (in terms of tonnage cast). Continuous casting of slabs with either in-line hot rolling or subsequent separate hot rolling are important post processing steps to produce coils of sheet. Slabs are typically cast from 150 to 500 mm thick and then allowed to cool to room temperature. Subsequent hot rolling of the slabs after preheating in tunnel furnaces is done in several stages through both roughing and hot rolling mills to get down to thickness's typically from 2 to 10 mm in thickness. Continuous casting with an as-cast thickness of 20 to 150 mm is called Thin Slab Casting (FIG. 2). It has in-line hot rolling in a number of steps in sequence to get down to thicknesses typically from 2 to 10 mm. There are many variations of this technique such as casting between of 100 to 300 mm in thickness to produce intermediate thickness slabs which are subsequently hot rolled. Additionally, other casting processes are known including single and double belt cast processes which produce as-cast thickness in the range of 5 to 100 mm in thickness and which are usually in-line hot rolled to reduce the gauge thickness to targeted levels for coil production. In the automotive industry, the forming of parts from sheet materials coming from coils is accomplished through many processes including bending, hot and cold press forming, drawing, or further shape rolling.